Mutations of parkin represent the most frequent cause of recessively-inherited Parkinson's disease (PD). Although parkin is expressed in many tissues including the brain, its mutations are linked to specific degeneration of nigral dopaminergic (DA) neurons and Parkinson's disease. Our long-term goal is to understand the function of parkin and its role in the selective degeneration of DA neurons in Parkinson's disease. Studies using parkin knockout mice and flies showed that loss of parkin disrupts mitochondrial functions. On the other hand, research on PD neurotoxins has demonstrated a critical role of mitochondrial dysfunction in PD pathogenesis. Thus, we would be able to gain significant insights into the molecular mechanism of Parkinson's disease by studying how parkin affects mitochondrial functions. Our previous studies have shown that parkin attenuates the toxicity of cytosolic dopamine by suppressing the transcription of monoamine oxidases (MAO), which are mitochondrial enzymes responsible for the oxidative deamination of dopamine. Our preliminary studies showed that parkin interacted with the transcription factor Estrogen-Related Receptor 1 (ERR1), which plays a significant role in transcription regulation of many mitochondrial proteins including MAO. Furthermore, we found that expression of COX4i2, isoforms 2 of subunit 4 of cytochrome C oxidase (a.k.a. Complex IV), was suppressed by parkin in a similar manner. Our microarray studies also showed that parkin altered the expression of many mitochondrial proteins encoded by the nuclear genome. Based on these lines of evidence, we hypothesize that parkin regulates the transcription of many nuclear-encoded mitochondrial proteins by interacting with transcription factors critically involved in mitochondrial gene expression. We will test this hypothesis by studying how parkin regulates the expression of monoamine oxidases and COX4i2, and examining the effects of parkin on the expression of other mitochondrial proteins. To investigate the cellular functions of parkin both in vitro and in vivo, we will use cell lines, parkin transgenic mice, parkin knockout mice, and cells derived from PD patients with parkin mutations. Knowledge gained from this study will significantly advance our understanding on how parkin affects mitochondrial functions and how such regulation goes awry when parkin is mutated. It will provide novel insights into the mechanisms by which mutations of parkin cause Parkinson's disease.